If you are struggling with above mentioned errors and have already gone through your web.config file. Also repaired/installed .NET framework (any version) with service pack (any version). Also tried with repairing/installing ajax toolkit (any version). And still not getting rid of these errors then you might want to try the following. It worked for me!
My local machine worked very well when I was developing an ajax enalbed web site. However, as soon as I put it on production server (Win Server 2003, IIS 6.0 with .NET Framework 3.5 and service pack 1), it started showing errors: Ajax client-side framework failed to load, 'Sys' is undefined etc. However, web site worked but Ajax functionality never worked.
After long effort and going through various docs. I found that I needed to add .axd file extension mapping on IIS. So on my IIS manager I right clicked on my web site; then on the dialogue box I clicked on virtual directory and then clicked on "Configuration" button. On the new dialogue box I clicke on "Add" and tried to add .axd extension which points to the aspnet_isapi.dll. The following is locaton of DLL on my machine. So I added the following path and clicked "OK"
And guess what the result was. The great relief! I spent more than 5 hours. However, at the end of the day I was happy that my ajax enalbed web site was up and running.
I am posting it here and hoping that it might help someone who is facing similar problems.
Have a great computing!